Mixer circuits



Dec. 6, 1955 R. l. BOWEN 2,725,327

MIXER CIRCUITS Filed Jan. 27, 1955 /A/ VENTO/2 ROBE/2T I Bows/v ATT RNEY United States Patent C 1 2,726,327 MIXER CIRCUITS Robert I. Bowen, Waltham, Mass., assigner to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application January 27, 1953, Serial No. 333,455 2 Claims. (Cl. Z50-20) This invention relates to an improved mixer circuit and more particularly relates to a triode-heptode mixer circuit of relatively high conversion gain.

Many triode-heptode mixer tubes are designed to utilize the triode section as an oscillator. Such tubes, however, may be used in circuits having a separate local oscillator tube whose output is applied to the injection grid or triode grid of the mixer. ln this case, the triode section is not used as an oscillator and the triode plate is usually grounded or left oating. The conversion gain of a mixer used in this manner is relatively low.

In accordance with this invention, the plate of the triode section of the mixer tube is connected to the heptode screen and, therefore, is at a positive potential relative to the iilament. Because of the increased total tube current flowing through the injection grid, as a result of the positive potential of the triode plate, the conversion gain of the mixer is considerably increased.

The drawing shows a schematic diagram of a mixer circuit according to the subject invention.

In the drawing, an electron discharge device of the well-known triode-heptode type includes an envelope containing a heptode section 11 and a triode section 12. The heptode section comprises a lament 14 which is common to both sections of the discharge device and a #l grid 15 adjacent said filament. The #l grid or injection grid vis also common to both sections of the discharge device and may, for example, be an oval grid surrounding said Iilament. The heptode section further includes a #2 grid 16, a #3 grid 17, a #4 grid 18, a #5 grid 19 and an anode 20, in the order named. Although shown by way of example as a heptode, section 11 of the electron discharge device 10 may contain any number of grids so long as the input grid receptive of input from source 2 is effectively shielded from the grid of triode section 12.

A local oscillator voltage from a separate oscillator 1 is aplied to the #l grid 15 which functions as an injector grid of both triode and heptode sections.

An incoming signal from an R. F. source 2, such as the R. F. section of a superheterodyne receiver, is applied to the #3 grid or signal grid 17 of the heptode section 11. The #3 grid 17 is shielded from the plate 20 and from #l grid 15 by screen grids 16 and 18, both of which are held at ground R. F. potential by means of by-pass capacitor 22. The #2 grid 16 also serves to shield the injection grid (#1 grid) 15 from the input signal grid 17. Since the input grid 15 of both triode section 12 and heptode section 11 receptive of the signal from local oscillator 10 is in the electron stream between the common cathode 14 and the plate circuit of heptode section 11, the voltages applied to the input grids 15 and 17 each modulate the electron stream, thus producing a beat note or intermediate frequency in the plate circuit of heptode section 11. The #5 grid 19 is a suppressor grid and may be connected directly to the lament 14 by a connection internally of the envelope, as shown in the drawing. The power supply for the lament may be any source of direct current voltage and is shown by way of example as a battery 24. An R. F. choke isolates the tilament power supply from R. F. currents. Capacitor 26 is a. filter capacitor for lament 14 and capacitor 27 is a decoupling capacitor.

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The heptode plate 20 is connected to the positive terminal B+ of a direct current plate supply through the plate resistor 29 and a tuned circuit 30 comprising a coil 31 shunted by a capacitor 32 and tuned to the desired diiierence of I. F. frequency. The output from tuned circuit 30 is coupled in the usual manner through capacitor 34 to an output circuit, such as an intermediate frequency amplier.

The plate 35 of triode section 12 is connected to the screen grids of heptode section 11 and is thus at a potential approximating that of the plate supply. Plate 35 may be connected to any point which is positive with respect to the potential of cathode 14. Although operation of tube 16 as a mixer is possible with plate 35 of triode section 12 iioating or tied to a reference point in the circuit such as ground, owing to the fact that the local oscillator signal is applied to grid 15 which is a part of heptode section 11, and hence, exerts an effect upon the electron stream of the heptode to whose plate circuit the I. F. output circuit is connected, a greatly increased conversion gain can be obtained with mixer tube iti if plate 35 is maintained positive with respect to the cathode.

In a typical example using a 2G21 tube, the xed voltage on the plate 2t) and screen grids 16 and 18 of the heptode section, as well as the voltage on the triode plate 35, were each approximately forty-ve volts and the filament voltage approximately 1.25 volts. The values of the injector grid resistor 38 and input signal grid resistor 39 were twenty-two kilohms and 470 kilohms, respectively, while plate supply resistor 29 was eighteen kilohms. It should be understood that these values are merely illustrative and depend upon the type of tube, used as well as upon the particular circuit constants of the circuit in which the mixer is to be used.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate wiht the scope of the invention within the art.

What is claimed is:

l. A frequency converter comprising a multisection tube including a rst section and a second section, said Iirst section having at least an electron emissive electrode, irst and second input grids, a screen grid and an output electrode, said second section including said electron emissive electrode, said rst input grid and an anode, rst means for supplying electromagnetic energy of a first vfrequency to said first input grid, second means for supplying electromagnetic energy of a second frequency to said second input grid, said anode being directly connected to said screen grid.

2. A frequency converter comprising a multisection tube including a first section and a second section, said rst section having at least an electron emissive electrode, rst and second input grids, a screen grid and an output electrode, said second section including said electron emissive electrode, said rst input grid and an anode, first means for supplying electromagnetic energy of a first frequency to said iirst input grid, second means for supplying electromagnetic energy of a second frequency to said second input grid, said anode being directly connected to said screen grid, and an output circuit connected to said output electrode and tuned to one of the conversion frequency products.

References Cited in the le of this patent UNITED STATES PATENTS 2,314,785 Holland Mar. 23, 1943 2,503,780 Van der Ziel Apr. 11, 1950 2,617,017 Dammers Nov. 4, 1952 

